As shown in FIG. 1, in a multi-hop relay system put forth by the IEEE 802.16j Standard Task Group, one or a plurality of relay stations (RS) are disposed between a multi-hop relay base station (MR-BS) and a mobile station (MS), and the extension of coverage and the increase of system capacity are thus realized through relay transmission of BS and terminal signals by the RS. In order to meet the requirements of different application scenarios, the RS may be set as a fixed RS or a mobile RS. Based on the situation whether prefixes and control messages are generated and transmitted, the RS may be classified into a transparent RS and a non-transparent RS. The MR-RS is called a RS for short hereinafter.
In a multi-hop relay network, a transmission link from a terminal to a BS via a RS comprises an access link and a relay link. Wherein, the access link is a communication link starting from or ending at the terminal and may be a link between the terminal and the BS directly connected to the terminal, or a link between the terminal and the RS connected to the terminal. The relay link is a link between the BS and the RS connected to the BS or a communication link between two RSs.
In centralized scheduling, the BS allocates resources to the access link and the relay link in a multi-hop relay cell in a unified way and generates a corresponding MAP (mapping message), or the RS generates its own MAP based on indication information of the BS. In distributed scheduling, the BS and the RS respectively determine resource allocation to sub-relay stations or terminals and generate a corresponding MAP. The relay network hereinafter refers to a non-transparent centralized scheduling multi-hop relay network.
Under the non-transparent circumstance prescribed in the IEEE 802.16j_D2 document, the frame structure of the BS and that of the RS respectively comprise an uplink subframe and a downlink subframe. Each uplink subframe and each downlink subframe respectively comprise an access zone and a relay zone. Wherein, the access zone is for access link transmission, and the relay zone is for relay link transmission. A relay MAP refers to a MAP message sent by the BS or the RS in the relay zone.
The IEEE 802.16j_D2 document specifies two path management modes: explicit path management and embedded path management. In the explicit path management, a path of a BS and an access RS is established and all CIDs (Connection ID) (including an RS management CID, a tunnel CID, a management tunnel CID, a terminal management CID and a terminal transport CID) on the path are associated with a path ID. A path ID/CID association list is generated by the BS and published to all RSs on the path. During data forwarding, the RS first obtains a CID from a MAP IE (MAP Information Element) header or an MAC PDU (Media Access Control Protocol Data Unit) header, and then gets a target station on a next hop through looking up in the path ID/CID association list. In the embedded path management, management CIDs of the RS are allocated by the BS according to a network topological structure system, and the CIDs conceal path information. During data transmission, the RS does not need to look up in the path ID/CID association list.
The IEEE 802.16j_D2 document defines four data transmission modes: a tunnel burst mode, a tunnel packet mode, a CID-based mode and a burst-based mode. Wherein the first two are tunnel transmission modes, and the last two are non-tunnel transmission modes. In the CID-based data transmission mode, an intermediate station reads the CID (the RS management CID, or the terminal management CID or the terminal transport CID) in the MAC PDU header, then looks up in the path ID/CID association list to obtain the information of the target station on the next hop, and performs data forwarding. There is no need to contain CID fields in the MAP IE. This transmission mode has small overhead of the MAP IE and broad application. In the published IEEE 802.16j_D2 document, the CID-based transmission mode is limited to being applicable to all application scenarios: a two-hop transparent mode, a two-hop non-transparent mode, and centralized scheduling and distributed scheduling in a multi-hop non-transparent mode; the tunnel packet mode is limited to being applicable to two-hop and multi-hop non-transparent modes; while due to its own technical defect, the tunnel burst mode is limited to being applicable to the distributed scheduling in the multi-hop non-transparent mode only.
In the CID-based data transmission mode, the RS gets path information through looking up in the path ID/CID association list and then forwards data by using the CID in the MAC PDU header. In the MAP IE, it may not contain CID information.
In the tunnel packet transmission mode, firstly the BS establishes a tunnel from the BS to an access RS and distributes a tunnel CID and a management tunnel CID for the tunnel. During downlink data transmission, the BS cascades the MAC PDUs sent to the terminals connected to a same access RS and encapsulates them with one relay MAC PDU header. The CID in the header is a tunnel CID or a management tunnel CID. During uplink data transmission, data encapsulation is performed by an access RS.
The tunnel packets on different tunnels may be packed into a same burst when they pass a same RS. The RS forwards data through the CID in the relay MAC PDU header, and the MAP IE may not contain CID information.
In the downlink tunnel burst transmission mode, the BS cascades the MAC PDUs sent to the terminals connected to a same access RS and put them into a burst. During uplink data transmission, data are processed by an access RS. The RS forwards data by using the CID in the MAP IE. The tunnel bursts on different tunnels shall not be packed into a same burst when they pass a same RS.
In the CID-based data transmission mode, as the CID field in the MAP IE is optional, the RS is unable to map the received data to the corresponding bandwidth. Moreover, when the delay between data reception and data forwarding of the RS is variable and unfixed, if the MAC PDU in a frame is lost, the RS will send the MAC PDU in a subsequent frame in advance. This circumstance will affect the performance of QoS (Quality of Service), jitter for example. Besides, if the condition of the down link of the RS is suddenly worsened, the BS will be unable to dynamically control the data transmission condition of the entire link.